The present invention relates to a fuel assembly, especially a fuel assembly for boiling water reactors.
A conventional fuel assembly for the boiling water reactor comprises a plurality of fuel rods, water rods arranged among the fuel rods, an upper tie plate and a lower tie plate both of which respectively supports each of the fuel rods and the water rods at their upper end and lower end, fuel spacers which maintain mutual intervals between the fuel rods and the water rods with a designated width, and a channel box surrounding a bundle of the fuel rods. The channel box is a long rectangular pipe made of zirconium. The channel box has such functions as (1) flowing a coolant in each of the fuel assembly uniformly, (2) forming a guide plane for both inserting and withdrawing operation of control rods among the fuel rods, and (3) maintaining rigidity and facilitating handling of the fuel assembly.
During operation of the nuclear reactor, the channel box deforms expanding outward by pressure difference between interior and exterior of the channel box 2 and neutron irradiation. The previously described deformation has a possibility to cause troubles in both inserting and withdrawing operation of the control rod by reducing intervals between the channel box and the control rod. Further, the deformation has a possibility to disturb supply of the coolant among the fuel rods by increasing a gap between the channel box and the lower tie plate.
Accordingly, the deformation is conventionally suppressed by sufficiently thickening of the wall of the channel box and leakage flow is regulated by furnishing a plate spring between the channel box and the lower tie plate (FIGS. 1 and 2 in JP-A-54-124183 (1979)). And, there is a restriction in the period of use of the channel box, for example, recycling use of the channel box obtained by disassembling of spent fuels are avoided.
But, in view of neutron economy, reducing a volume fraction of structural material in the reactor core by thinning of the wall of the channel box as possible is preferable. Moreover, in view of reducing radioactive waste, using of the channel box in the reactor core as long as possible is preferable. Recycling use of used channel box is one of the methods for extensional use of the channel box (U.S. Pat. No. 4,988,476).
Currently, using of the fuel assembly to higher burn up has been progressing. Consequently, the channel box is also used for a longer period than a channel box of a conventional fuel assembly. The longer use of the channel box causes increasing of the deformation of the channel box. Further, lowered spring force of the plate spring for regulating the leakage flow between the channel box and the lower tie plate by extension of the using period has a possibility to deteriorate the previously described function of the leakage flow regulation.
Due to the above described background, structures of the channel boxes which are durable for long period usage by suppressing the deformation without much increasing of the volume of the channel box are disclosed in JP-B-1-13075 (1989), U.S. Pat. Nos. 4,749,543 and 4,749,544. In accordance with the above disclosed channel boxes, the wall thickness at corner portion where the intense stress is generated in the cross section by the pressure difference between interior and exterior of the channel box is thickened, and the wall thickness at other region where bears a relatively weak stress is thinned.
The structure of the channel box which is able to regulate the previously described leakage flow has a thick wall region only at the lower end portion of the channel box for increasing the rigidity as disclosed, for example, in U.S. Pat. No. 3,715,274. Consequently, the deformation at the lower end portion of the channel box is suppressed.
The channel box having thick wall region at the corner portion, thin wall region at middle of side wall portion between the corner portions, and thick wall region all through the circumference at the lower end portion is disclosed in JP-A-1-227991 (1989). The thick wall region at the lower end portion of the channel box occupies a space from the lower end of the channel box to a position fitting with the lower tie plate, that is, the same level as the upside of the lower tie plate.